In a known conventional technology, a long length of tape-shaped wire is formed from oxide superconductor, e.g., a Bi2223 phase, by a powder-in-tube method. In this method, a raw material powder of a superconducting phase is filled in a metal pipe made of silver or the like. The resulting metal pipe filled with the raw material powder is subjected to wire drawing, so that a clad wire is produced. A plurality of clad wires are bound and inserted into a metal pipe made of silver or the like, and the wire drawing is performed, so that a multifilament wire is produced. The resulting multifilament wire is subjected to rolling, so that a tape shaped wire is produced. The tape shaped wire is subjected to a first heat treatment, so that the desired superconducting phase is generated. Subsequently, the resulting tape shaped wire is rolled again and, thereafter, is subjected to a secondary heat treatment, so that crystal grains in the superconducting phase are mutually bound. These two-time plastic deformation and heat treatment are performed generally in an atmosphere containing oxygen by 7 to 21 volume percent, although these may simply be performed once. Consequently, a tape shaped wire including a plurality of superconducting filaments in a metal sheath is produced.
However, there is a drawback in the conventional technology: that is, the residual gases in the inside of the raw material powder expand during the above-described first and secondary heat treatments such that voids are generated between crystals of the superconductor, or the gases and the raw material powder are combined such that amorphous phases are precipitated, and consequently connection between crystals of the superconductor is hindered, which results in the decrease in critical current density. In addition, there is a problem in that gases gather locally and, thereby, cause defects, e.g., blisters.
Japanese Unexamined Patent Application Publication No. 6-342607 discloses that after wire drawing, heat treatment is performed at 550° C. to 760° C. in a depressurized atmosphere so that adsorbed gases are removed from a raw material powder. However, in this technology, since the heat treatment is performed after the wire drawing, the gas permeability is poor, because the density of the raw material powder in the metal pipe increases as a result of the wire drawing. Consequently, it is difficult to adequately perform the degasification treatment. Since the end portion of the metal pipe is not sealed under depressurization, air and the like may enter into the metal pipe through the end of the metal pipe after the heat treatment is completed.
Japanese Unexamined Patent Application Publication No. 6-176635 discloses that an oxide powder is filled in a metal pipe in a vacuum or in an atmosphere at a humidity of 30 percent or less. However, in this technology, air may remain in the metal pipe. The residual air expands during the stages of the above-described first heat treatment and secondary heat treatment. Consequently, blisters occur, or connection between crystals is hindered, resulting in decrease of the critical current density.
Furthermore, Japanese Unexamined Patent Application Publication No. 6-309967 discloses that a rod-shaped material made of an oxide superconductor powder is encapsulated under vacuum into a metal pipe under depressurization of 1/103 Torr (0.13 Pa) or less while heating in the range of 200° C. to 800° C. However, in this technique, degasification cannot be achieved adequately to the central portion of the metal pipe, because the gas permeability is poor, since the metal pipe is filled with the rod-shaped material, and not with a powder. The rod-shaped material may be abnormally deformed during a wire drawing step such that voids or the like are generated in the metal pipe, thereby decreasing the critical current density. The higher the heating temperature, the more the gases can be exhausted. However, adequate degasification cannot be achieved because the maximum temperature of possible heating is only about 700° C. to 750° C., since the powder may be decomposed if heated to 800° C. in a depressurized atmosphere of 1/103 Torr or less.